1. Field of the Invention
This invention relates to systems which measure the duty cycle of a periodic signal. More particularly, this invention relates to a circuit and various components which measure the duty cycle of a periodic signal with great accuracy.
2. Description of the Related Art
The duty cycle, in electronic signals, is often expressed as a percentage of duration of a 1-level compared to the total duration of a complete 1-0 cycle. The duty cycle is an expression of the average DC energy of a clocking signal introduced by the duty cycle. A 50% duty cycle has average DC energy of one-half the difference of the 1-level and the 0-level as there is an equal duration of 1-level signal and 0-level signal. The duty cycle in clocking waveforms is also a measure of second harmonic energy in a clocking signal which can appear as a deterministic jitter source from odd-even pulse widths in the clocking signal. Both interpretations can lead to non-optimal performance of very high speed signal processing if the duty cycle deviates from tolerable levels. For this reason, means for measuring duty cycle to support corrective actions are needed.
During the production of many high-speed systems, special care and calibrations are done to make sure good duty cycles are present within the processing elements. In some systems, however, the application space is so broad, that it is not possible to use factory calibration information as it is not relevant to the then current environment. This is true, for example, where operation of a device is specified for a wide range of operating frequencies and a wide range of temperatures. In these cases, automatic measurement of the duty cycle at various spots within the electronic processing path must be supported. In this fashion, once the system is in the operating environment, the calibration procedure can be invoked to take corrective actions appropriate for the given environment without external probing or outside test equipment. For this to be practical, very high performance duty cycle measuring circuits must be implemented with a small amount of electronics and must be able to make high-quality duty cycle measurements in a short amount of time.
Past methods for measuring duty cycle typically involved using an oscilloscope device to observe the waveform shape and then, firmware in the oscilloscope would interpret the waveform data to compute the duty cycle number. This high-performance means of accurately measuring duty cycle requires expensive external test equipment, access to the testing points and a relatively constant operating environment suitable for reliably applying pre-computed corrective action based on relatively infrequent actual calibration information.
Cruder methods for measuring duty cycle are accomplished by measuring the DC voltage of a high-speed signal and then applying a low-pass filter to average the result. For well-shaped repetitive signal with binary 1 and 0 values encoded, the duty cycle value is given by comparing the DC1-level and DC 0-level to the average measured DC voltage. For example, if a 1-level was 1000 mV and a 0-level was 0 mV, then a measured averaged DC voltage of 500 mV (mid-way) might be thought to indicate a 50% duty cycle. The problem with this method is that it assumes well-shaped waveforms. If there is a variation in waveform shape for 1-levels and 0-levels (something that is common in modern high-speed electronics), for example, caused by variation in rise-time versus fall-time, then a 500 mV averaged DC voltage might have been caused by a condition other than a 50% duty cycle. That is, a 50% voltage average does not necessarily indicate a 50% duty cycle. As it is duty cycle, rather than average DC voltage, that cause deterministic jitter and other denigrating effects, a measure that directly indicated duty cycle is required.
Making assumptions about the shape of the wave form results in inaccuracies. In order to increase accuracy, what is needed, and is not found in the prior art, is a means of assuring an accurate shape for the waveform. One way of assuring accuracy has been found to be via the use of a digital sampling device. The output of the digital sampling device is controlled so that the shape is predictable and accurate. At that point, the desired waveform is produced with accuracy and averaging is done without error.
What is also needed is accurate measurement of the duty cycle using a clocking signal having minimal hardware.